The present invention relates to an apparatus for measuring the thickness of films such as plastic films, wherein such measurements are made by detecting capacitance variations when the film is placed in close proximity to a sensing head. Variations in film thickness cause corresponding dielectric variations which are detected as variations in measured capacitance.
The use of capacitance measuring sensors for measuring film thickness is known in the art. One sensor is disclosed in U.S. Pat. No. 3,764,899, owned by the assignee of the present invention. This patent discloses that film thickness may be measured by passing the film through a capacitance sensor, wherein variations in film thickness are sensed as dielectric thickness variations between the capacitance members and are detected as variations in a signal which is applied to the capacitance members.
Another capacitance measuring device is disclosed in British Patent No. 2038483, also owned by the assignee of the present invention. This patent is directed to a capacitance sensor for sensing film thickness changes wherein the film is passed over capacitance elements arranged in a side-by-side relationship. Variations in film thickness are detected as variations in a signal applied to an electrode positioned in an air gap over which the film is moved. A plurality of passages are formed in the sensor.
Still another capacitance measuring device is disclosed in pending U.S. Patent Application Ser. No. 341,493, owned as well by the assignee of the present invention. That patent application is directed to an improvement over the aforementioned British patent, wherein the invention is constructed to solve the problem and minimize the effects of temperature changes on the capacitance measurements, both in terms of temperature effects causing relative expansion and contraction of the dimension of the measuring components, and also in terms of the temperature effects causing relative changes in the dielectrics found between capacitance members.
While this prior art represents significant advances in obtaining accurate measurements of film thickness by detecting capacitance variations when a film is placed in proximity to a sensor, there is a problem which has not been solved. Specifically, applicants have discovered that contact between a film and the sensor is absolutely essential to achieve thickness readings with acceptable margins of error. This is a particularly difficult problem in the on-line application of capacitance sensors when ultra-repeatability is needed.
There are several potential sources or causes for loss of contact between the sensor and a film. Film supporting or conveying systems may cause loss of contact. For example, if rollers supporting the film are not precisely parallel the film tension may change and contact with the sensor head may be lost. The dimensions or diameter of the film rollers may vary thereby causing the film to be supported at different levels across the extent of the film. Additionally, the suspension of the guides or rollers carrying the film, even down to the bearings upon which the rollers rotate, may have an effect on the film tension and thus, contact with the sensor.
A second source of lost or varying sensor to film contact is that the film may be wrinkled. Even slight wrinkling can cause variances in the film contact with the sensor and thus cause fluctuations in the sensitivity and accuracy of measurements.
Even if film rollers and guides are machined to very precise tolerances and even if extremely stringent quality control is exercised so that the film emerges from the die without a wrinkle, there is still another cause of loss of contact between the film and the sensor. That is, applicants have noted that as the sensor head approaches the edges of the in-process film, the film tension may change causing the film to lose contact with the sensor head.
During testing of capacitance sensors, it has been noted that without a solution to this contact problem, e.g., additional expensive precise tooling of processing equipment, the practical monitoring of in-process films may be unworkable. This is particularly true when thin films, for example with a thickness of below 50 mils, are being assessed.
In the case of thin films, the influences of lost contact are bigger because the relative part of the stray field is bigger. Typical sensitivity for a 20 mil cast film is in the range of 15% for 0.1 mm of distance between the sensing head and the film.
By way of contrast, in the case of thicker films, the electrical field at the top of the sensor is concentrated or guided by the film itself; the relative part of the stray field is smaller. Therefore, the influences on the field by position changes away from the top of the sensor are smaller. Typical sensitivities if contact with the sensor is lost are in the range of 10% per 0.1 millimeter of distance (with a 150 micron material).
Thus, one of the biggest problems in the measurement of variations in film thickness using capacitance variations is keeping the film, particularly thin film, in contact with the sensor head all across the film surface and as the film is being processed.